Men who cheat on their spouses have always enjoyed an expedient explanation: Evolution made me do it. Many articles (here is one, and here is another), especially in recent years, have explored the theory that men sleep around because evolution has programmed them to seek fertile (and, conveniently, younger) wombs.

But what about women? If it's really true that evolution can cause a man to risk his marriage, what effect does that have on women's sexuality?

A new journal article suggests that evolutionary forces also push women to be more sexual, although in unexpected ways. University of Texas psychologist David Buss wrote the article, which appears in the July issue of Personality and Individual Differences, with the help of three graduate students, Judith Easton (who is listed as lead author), Jaime Confer and Cari Goetz. Buss, Easton and their colleagues found that women in their 30s and early 40s are significantly more sexual than younger women. Women ages 27 through 45 report not only having more sexual fantasies (and more intense sexual fantasies) than women ages 18 through 26 but also having more sex, period. And they are more willing than younger women to have casual sex, even one-night stands. In other words, despite the girls-gone-wild image of promiscuous college women, it is women in their middle years who are America's most sexually industrious. (See the top 10 political sex scandals.)

By contrast, men's sexual interest and output, usually measured by a reported number of orgasms per week, peaks in the teen years and then settles to a steady level (an average of three orgasms per week) for most of their lives. As I pointed out in March, most men remain sexually active into their 70s. According to the new study, as well as the study I wrote about in March, women's sexual ardor declines precipitously after menopause.

Why would women be more sexually active in their middle years than in their teens and 20s? Buss and his students say evolution has encouraged women to be more sexually active as their fertility begins to decline and as menopause approaches.

Here's how their theory works:

Our female ancestors grew accustomed to watching many of their children  perhaps as many as half  die of various diseases, starvation, warfare and so on before being able to have kids of their own. This trauma left a psychological imprint to bear as many children as possible. Becoming pregnant is much easier for women and girls in their teens and early 20s  so much easier that they need not spend much time having sex. (See photos of the history of the cougar.)

However, after the mid-20s, the lizard-brain impulse to have more kids faces a stark reality: it's harder and harder to get pregnant as a woman's remaining eggs age. And so women in their middle years respond by seeking more and more sex.

To test this theory, Buss and his students asked 827 women to complete questionnaires about their sexual habits. And, indeed, they found that women who had passed their peak fertility years but not quite reached menopause were the most sexually active. This age group  27 through 45  reported having significantly more sex than the two other age groups in the study, 18 through 26 and 46 and up. Women in their middle years were also more likely than the younger women to fantasize about someone other than their current partner. The new findings are consistent with those of an earlier Buss paper, from 2002, which found that women in their early 30s feel more lustful and report less abstinence than women in other age groups. In both studies, these findings held true for both partnered and single women, meaning that married women in their 30s and early 40s tend to have more sex than married women in their early 20s; ditto for single women. Also, whether the women were mothers didn't matter. Only age had a strong affect on women's reported sexual interest and behavior. (Read about cougar cruises.)

And yet there are a few flaws with the data in the new paper. Chiefly: some three-quarters of the participants in the study were recruited on Craigslist, a website where many go to seek hookups, meaning there was a self-selection problem with the sample. (The other participants were students at the University of Texas in Austin.) The authors also note that there are some alternative explanations for why women in their 30s and early 40s might be more sexual. Many of them may simply be more comfortable with sex than women in their teens and early 20s. Still, that raises the question of why they are more comfortable: perhaps evolution programmed that comfort.

Buss is the author of The Evolution of Desire: Strategies of Human Mating, now in its fourth edition, and has become associated with evolutionary explanations for sexual behavior. His theories help explain why men can be cads  and why women can be cougars.